Optical coherence tomography (OCT) is an established medical imaging technique that relies on light for producing an image. In OCT, light from a broadband light source is split by an optical fiber splitter with one fiber directing light to a sample path and the other fiber directing light to a reference path mirror. An end of the sample path is typically connected to a scanning device. The light reflected from the scanning device is recombined with the signal from the reference mirror to form interference fringes, which are transformed into a depth resolved image.
Numerous different OCT techniques have been developed, including swept source OCT. In swept source OCT, a narrowband light source is rapidly tuned over a broad optical bandwidth, and spectral components are encoded in time. Thus, image quality in swept source OCT relies on the swept laser source achieving very narrow bandwidths at very high frequencies (e.g., 20-200 kHz) over a very short period of time (e.g., 10,000-10,000,000 Sweeps/sec).
A typical set-up for a swept source OCT system uses a ring resonator that includes an optical amplifier, a tunable filter, and an optical coupler. Laser light at a specific bandwidth is produced by the optical amplifier and sent through the filter. The filter assists in maintaining the outputted light at the specific bandwidth. From the filter, the light travels to the optical coupler where a portion of the light is directed to an interferometer and the remainder of the light returns to the optical amplifier. The process repeats to achieve different bandwidths of light.
A problem with this set-up is that a single pass through an optical amplifier cannot impart enough energy to the light to obtain the very narrow bandwidths in an optimal amount of time. Another problem with this set-up is that a single pass through an optical amplifier cannot impart enough energy to the light to prevent broadening of the obtained bandwidth, particularly at higher frequencies. Both problems lead to degradation of image quality.